1. Field of the Invention
The present invention relates to a rotation angle detecting device for detecting a rotation angle of an object to be measured and, more particularly, to a throttle opening detecting device for detecting a rotation angle of a throttle valve for regulating the amount of intake air sucked into a cylinder of an internal combustion engine.
2. Description of the Related Art
Conventionally, as a rotation angle detecting device for detecting a rotation angle of an object to be measured, for example, a throttle opening sensor (also referred to as a throttle position sensor) for detecting the degree of opening of a throttle valve (throttle opening) of an internal combustion engine has been proposed. One such example is disclosed in Japanese Patent document JP 2001-317909 A. This is, as shown in FIGS. 8A and 8B, such that a rotating shaft 101 of an object to be measured such as a throttle valve (not shown) is rotatably supported by a housing 103 through a bearing 102. A cylindrical rotor core (corresponding to a yoke) 104 is fixed to one of the ends of the rotating shaft 101. On an inner circumferential side of the rotor core 104, a columnar stator core 105 is coaxially arranged. A magnet 107 is fitted into each of two notches 106 in the rotor core 104 so as to be fixed thereto. Each of the two magnets 107 is formed to have a planar or columnar shape. On both end faces thereof, an N-pole and an S-pole are magnetized in parallel. An inner circumferential face of the rotor core 104 is opposed to an outer circumferential face of the stator core 105 through a small air gap therebetween except for the vicinity of each of the magnets 107. On the other hand, a magnetic detection gap 109 having a constant width for forming a parallel magnetic field is formed in the middle of the stator core 105 so as to penetrate therethrough in a diameter direction. Two Hall ICs 110 are horizontally arranged in the magnetic detection gap 109.
Since the two magnets 107 are arranged at the opposed positions in the diameter direction of the rotor core 104 so as to repel each other in the throttle opening sensor configured as described above, magnetic flux generated from the N-pole of each of the magnets 107 passes through a magnetic path from the rotor core 104, the stator core 105, the magnetic detection gap 109 (the Hall ICs 110), the stator core 105 to the rotor core 104 so as to return to the S-pole of each of the magnets 107. When the rotor core 104 rotates with the rotation of the object to be measured such as the throttle valve, a density of magnetic flux passing through the magnetic detection gap 109 in the stator core 105 (a magnetic flux density crossing the Hall ICs 110) varies in accordance with its rotation angle. In accordance with the magnetic flux density, an output voltage from the Hall ICs 110 varies. In the throttle opening sensor shown in FIG. 8A a relatively large air gap 111 is formed in the vicinity of each of the magnets 107 on the inner circumferential side of the rotor core 104. As a result, short-circuit of the magnetic flux between both poles of each of the magnets 107 and the stator core 105 can be prevented by the air gap 111, thereby preventing the density of the magnetic flux passing through the magnetic detection gap 109 (the Hall ICs 110) from being lowered.
Moreover, as shown in FIGS. 6A and 9, a rotation angle sensor retaining the Hall IC 110 in a magnetic detection gap 122 formed between retaining pieces 121 of divided-type stator cores 120 has been proposed in U.S. Pat. No. 6,707,292 B2. When a rectangular parallelepiped magnet 130 rotates with the rotation of an object to be measured, a density of magnetic flux passing through the magnetic detection gap 122 (a density of magnetic flux crossing the Hall IC 110) changes in accordance with its rotation angle. In accordance with the density of the magnetic flux, an output voltage of the Hall IC 110 changes. Each of the stator cores 120 includes a shoulder part 123 extended from a lower end of the retaining piece 121 in the drawing to a horizontal direction in the drawing; a bent part 124 obtained by bending at an end of the shoulder part 123; and an extended part 125 extended from an end of the bent part 124 in a straight manner to the lower end in the drawing.
In the throttle opening sensor described in Japanese Patent document JP 2001-317909 A, however, and as shown in FIGS. 8A and 8B, the Hall ICs 110 are held in a connector housing 114 obtained by resin molding of a terminal 112, to which lead wirings of the Hall ICs 110 are connected, the stator core 105, the spacer 113 and the like. Specifically, since the housing 103 for rotatably retaining the rotor core 104 and the two magnets 107 and the connector housing 114 for retaining the stator core 105 and the Hall ICs 110 are constituted by separate components, the positional accuracy (combination accuracy) of the stator core 105 and the Hall ICs 110 with respect to a magnetization direction of the two magnets 107 can hardly be obtained. Therefore, a variation in output from the Hall ICs 110 is likely to occur. As a result, there arises a problem that detection accuracy of the rotation angle of the magnets 107 rotating with the rotation of the object to be measured is lowered. Moreover, since two magnets 107 are provided as magnetic field sources, the number of components and the number of assembly steps are increased, resulting in a problem of increased cost.
Moreover, in the rotation angle sensor described in U.S. Pat. No. 6,707,292 B2, and as shown in FIGS. 6A and 9, while the rotation angle of the magnet 130 changes from the minimum angle (for example, 0°) to the vicinity of the maximum angle (for example, 80°), an output from the Hall IC 110 changes in accordance with a change in density of the magnetic flux passing through the magnetic detection gap 122. Each of the stator cores 120 includes the straight extended part 125 forming an air gap with both end faces of the magnet 130 when the magnet 130 rotates at a large rotation angle. Therefore, an output with a convex profile having an inflection point in the vicinity of the maximum angle is generated, rather than an ideal output, as indicated with a solid line in a graph of FIG. 10 presented herein according to the present invention. More specifically, a difference between the output from the Hall IC 110 and the ideal output becomes the largest when the rotation angle of the magnet 130 is in the vicinity of 45°. As a result, there arises a problem that linearity of the output value of the Hall IC 110 with respect to the rotation angle of the magnet 130, which rotates with the rotation of the object to be measured, is degraded within the range of the detected angle of the object to be measured.